This invention relates to a printing devices and more particularly to a developer or toner delivery system for presenting developer or toner to a charge retentive surface or an electronically addressable printhead utilized for depositing developer in image configuration on plain paper substrates.
Of the various electrostatic printing techniques, the most familiar is that of xerography wherein latent electrostatic images formed on a charge retentive surface are developed by a suitable toner material to render the images visible, the images being subsequently transferred to plain paper.
A less familiar form of electrostatic printing is what has come to be known as direct electrostatic printing (DEP). This form of printing differs from the aforementioned xerographic form, in that, the toner or developing material is deposited directly onto a plain (i.e. not specially treated) copy substrate in image configuration. This type of printing device is disclosed in U.S. Pat. No. 3,689,935 issued Sept. 5, 1972 to Gerald L. Pressman et al.
In xerographic printing systems, the latent images on a charge retentive surface can be rendered visible by the use of magnetic and fiber brushes, the former of which results in toner being presented to the charge retentive surface which has a fairly good charge distribution but toner particles having a low charge level are not avoided. The problem of charge distribution of the toner and the charge level has been addressed in copending application U.S. Ser. No. 926,169 assigned to the same assignee as this application. As disclosed therein, a developer or toner delivery system disposed to one side of a printhead includes a conventional magnetic brush supported for rotation adjacent a supply of developer contained in a hopper.
A developer donor roll is supported for rotation intermediate the magnetic brush and the printhead structure. The donor roll structure is spaced from the printhead approximately 0.006 inch. The magnetic brush has a DC bias of about 100 volts applied thereto via a DC voltage source. An AC voltage of about 400 volts is applied to the donor roll creates a localized field between the donor roll and the printhead causing toner to jump to the vicinity of apertures in the printhead.
Traveling wave devices have been employed for delivering particulate material to a charge retentive surface, for example, U.S. Pat. No. 3,872,361 issued to Masuda which discloses an apparatus in which the flow of particulate material along a defined path is controlled electrodynamically by means of elongated electrodes curved concentrically to a path, as axially spaced rings or interwound spirals. Each electrode is axially spaced from its neighbors by a distance about equal to its diameter and is connected with one terminal of a multi-phase alternating high voltage source. Adjacent electrodes along the path are conncted with different terminals in a regular sequence, producing a wave-like, non-uniform electric field that repels electrically charged particles axially inwardly and tends to propel them along the path.
U.S. Pat. No. 3,778,678 also issued to Masuda relates to a similar device as that disclosed in the aforementioned U.S. Pat. No. 3,872,361.
U.S. Pat. No. 3,801,869 issued to Masuda discloses a booth in which electrically charged particulate material is sprayed onto a workpiece having an opposite charge, so that the particles are electrostatically attracted to the workpiece. All of the walls that confront the workpiece are made of electrically insulating material. A gridlike arrangement of parallel, spaced apart electrodes, insulated from each other extends across the entire area of every wall, parallel to a surface of the wall and in intimate juxtaposition thereto. Each electrode is connected with one terminal of an alternating high voltage source, every electrode with a different terminal than each of the electrodes laterally adjacent to it, to produce a constantly varying field that electrodynamically repels particles from the wall. While the primary purpose of the device disclosed is for powder painting, it is contended therein that it can be used for electrostatic printing.
The Masuda devices all utilize a relatively high voltage source (i.e. 5-10 KV) operated at a relatively low frequency, i.e. 50 Hz, for generating his traveling waves. Another commonality among the Masuda et al devices is the nonrecognition of the problem caused by toner with a wide charge distribution.
The movement of toner via traveling waves for use in a xerographic development system is also disclosed in U.S. patent application Ser. No. 374,376 filed on May 3, 1982, now abandoned. In that application, there is disclosed a device comprising an elongated conduit which utilizes traveling waves for transporting toner from a supply bottle to a toner hopper.
The performance of xerographic printing systems based on electrostatic monopole force, broadly referred to as Charged Pigment Xerography (CPX), is strongly dependent on the distribution of charge on the toner and especially requires the avoidance of low (i.e. has the opposite sign from the electrostatic image) and wrong sign toner (i.e. toner that is charged to the same sign as the image to be developed) The performance of direct electrostatic printing systems also have the foregoing requirements.
Direct electrostatic printers are known in the art. For example, Pressman et al in U.S. Pat. No. 3,689,935 disclose an electrostatic line printer incorporating a multilayered particle modulator or printhead comprising a layer of insulating material, a continuous layer of conducting material on one side of the insulating layer and a segmented layer of conducting material on the other side of the insulating layer. At least one row of apertures is formed through the multilayered particle modulator. Each segment of the segmented layer of the conductive material is formed around a portion of an aperture and is insulatively isolated from every other segment of the segmented conductive layer. Selected potentials are applied to each of the segments of the segmented conductive layer while a fixed potential is applied to the continuous conductive layer.
An overall applied field projects charged particles through the row of apertures of the particle modulator and the density of the particle stream is modulated according to the the pattern of potentials applied to the segments of the segmented conductive layer. The modulated stream of charged particles impinge upon a print-receiving medium interposed in the modulated particle stream and translated relative to the particle modulator to provide line-by-line scan printing. In the Pressman et al device, the supply of the toner to the control member is not uniformly effected and irregularities are liable to occur in the image on the image receiving member. High-speed recording is difficult and moreover, the openings in the printhead are liable to be clogged by the toner. For these reasons, this method has not yet been put into practical use.
U.S. Pat. No. 4,491,855 issued on Jan. 1, 1985 in the name of Fujii, et al discloses a method and apparatus utilizing a controller having a plurality of openings or slit-like openings to control the passage of charged particles and to record a visible image by the charged particles directly on an image receiving member. Specifically disclosed therein is an improved device for supplying the charged particles to a control electrode and has made high-speed and stable recording possible. The improvement in Fujii et al lies in that the charged particles are supported on a supporting member and an alternating electric field is applied between the supporting member and the control electrode. Thus, as alleged therein, it has become possible to sufficiently supply the charged particles to the control electrode without scattering them.
U.S. Pat. No. 4,568,955 issued on Feb. 4, 1986 to Hosoya et al discloses a recording apparatus wherein a visible image based on image information is formed on an ordinary sheet by a developer. The recording apparatus comprises a developing roller spaced at a predetermined distance from and facing the ordinary sheet and carrying the developer thereon, a recording electrode and a signal source connected thereto, for propelling the developer on the developing roller to the ordinary sheet by generating an electric field between the ordinary sheet and the developing roller according to the image information, a plurality of mutually insulated electrodes provided on the developing roller and extending therefrom in one direction, an AC and a DC source are connected to the electrodes, for generating an alternating electric field between adjacent ones of the electrodes to cause oscillations of the developer found between the adjacent electrodes along electric lines of force therebetween to thereby liberate the developer from the developing roller. In a modified form of the Hosoya et al device, a toner reservoir is disposed beneath a recording electrode which has a top provided with an opening facing the recording electrode and an inclined bottom for holding a quantity of toner. In the toner reservoir are disposed a toner carrying plate as the developer carrying member, secured in a position such that it faces the end of the recording electrode at a predetermined distance therefrom, and a toner agitator for agitating the toner.
The toner carrying plate is made of an insulator. The toner carrying plate has a horizontal portion, a vertical portion descending from the right end of the horizontal portion and an inclined portion downwardly inclining from the left end of the horizontal portion. The lower end of the inclined portion is found near the lower end of the inclined bottom of the toner reservoir and immersed in the toner therein. The lower end of the vertical portion is found near the upper end of the inclined portion and about the toner in the reservoir.
The surface of the toner carrying plate is provided with a plurality of uniformly spaced parallel linear electrodes extending in the width direction of the toner carrying plate. At least three AC voltages of different phases are applied to the electrodes. The three-phase AC voltage source provides three-phase AC voltages 120 degrees out of phase from one another. The terminals are connected to the electrodes in such a manner that when the three-phase AC voltages are applied, a propagating alternating electric field is generated, which propagates along the surface of the toner carrying plate from the inclined portion to the horizontal portion.
The toner which is always present on the surface of lower end of the inclined portion of the toner carrying plate is negatively charged by friction with the surface of the toner carrying plate and by the agitator. When the propagating alternating electric field is generated by the three-phase AC voltages applied to the electrodes the toner is transported up the inclined portion of the toner carrying plate while it is oscillated and liberated to be rendered into the form of smoke between adjacent linear electrodes. Eventually, it reaches the horizontal portion and proceeds. When it reaches a development zone facing the recording electrode it is supplied through the opening to the ordinary sheet as recording medium, whereby a visible image is formed. The toner which has not contributed to the formation of the visible image, is carried along such as to fall along the vertical portion and then slide down into the bottom of the toner reservoir by the gravitational force to return to a zone, in which the lower end of the inclined portion of the toner.
Notwithstanding the advancements made in direct electrostatic printing, there is still need for improvement in the quality of the toner delivered to the substrate on which the images are formed. Specifically, a delivery system capable of delivering a high percentage of well charged (i.e. greater than 1 micron/gm) toner of the proper sign is highly desirable.
It is known to remove contaminants such as debris prior to the use of the developer for its intended purpose. Such an arrangement is disclosed in U.S. patent application Ser. No. 718,615, now U.S. Pat. No. 4,639,115 wherein a biased roller is disposed in the developer housing at a location suitable for removing debris such as paper fibers from the toner prior to use for developing the images. The foregoing application does not involve the type of printing herein contemplated nor does it suggest the type of toner delivery system disclosed and claimed herein. Its relevance is the altering of the composition of the toner by removing contaminants from the developer prior to image development.
It is also known to separate low charge residual toner from toner charged to the desired level. Such is disclosed in a residual toner removal apparatus in U.S. patent application Ser. No. 563,729. In that application a cylindrically shaped member comprising an electrostatic wave transport is adapted to move toner larger than a predetermined size and having a predetermined charge level along its longitudinal axis.
A charged toner conveyor is disclosed in copending U.S. patent application Ser. No. 614,499, now U.S. Pat. No. 4,647,179 field in the name of Fred Schmidlin on May 29, 1984 which application is assigned to the same assignee as the instant application. The device disclosed therein differs from the U.S. patent application Ser. No. 563,729 in that, toner separation is not effected